1. Field of the Invention
This invention relates to a process for producing rare earth-cobalt (RECo.sub.5) permanent magnets having a relatively high and stable intrinsic coercivity, and more particularly to a process for producing (Sm.sub.x Pr.sub.1-x)Coy (wherein y is 4.6-5.0) in which vvaried cooling rates in the temperature range between a sintering proceeding and an isothermal annealing proceedings are rendered so as to significantly improve the magnetic properties of the magnets.
2. Description of the Prior Art
There has been a rapid development of RECo.sub.5 permanent magnet materials since the 1970's. Among the known RECo.sub.5 permanent magnets, it has been found that NdCo.sub.5, PrCo.sub.5 and SmCo.sub.5 are in order, the top three RECo.sub.5 permanent magnets in terms of saturation magnetization value; specifically, their values are 12.3 kG, 12.0 kG and 10.7 kG respectively. Therefore, the top three theoretical values of the maximum energy product of the known RECo.sub.5 permanent magnets will also probably be those of NdCo.sub.5, PrCo.sub.5 and SmCo.sub.5. The accuracy of the theoretical values of the maximum energy product is dependent on the normal coercivity of each of the permanent magnets. If the normal coercivity is less than a certain value. the theoretical values of the maximum energy product will not be accurate. In the meantime, the normal coercivity of the magnets will be decided by the crystal anisotropy field (Ha) of magnetic substances contained in each of the permanent magnets. The anisotropy fields of the NdCo.sub.5, PrCo.sub.5 and SmCo.sub.5 are 30 kOe, 145 kOe and 290 kOe respectively. Accordingly, the real maximum energy product of NdCo.sub.5 must be particularly less than the theoretical value thereof. On the other hand, the 10.7 kG value of the saturation magnetization renders the SmCo.sub.5 magnet having a high value of residual magnetic flux density. The 290 kOe of the anostropy field renders the SmCo.sub.5 magnet having a high coercivity and a high intrinsic coercivity. Thus, it may be said that the SmCo.sub.5 magnet has the optimum magnetic properties among the known RECo.sub.5 permanent magnets. However, due to the low supply of samarium in the earth, it is the expense that limits the application of the SmCo.sub.5 magnet. It becomes a problem to be eagerly solved in the art.
Initially, some experiments were conducted on the PrCo.sub.5 permanent magnet for two major reasons: (1) its saturation magnetization value is higher than that of the SmCo.sub.5 alloy in the RECo.sub.5 permanent magnets, and its anisotropy field half of that of the SmCo.sub.5 alloy; and (2) the amount of praseodymium in the earth is about five times of that of samarium. Nevertheless, it was soon found that the residual magnetic flux density of the PrCo.sub.5 alloy is higher than that of the SmCo.sub.5 alloy, while the coercivity as well as the intrinsic coercivity of the former are much less than those of the later. That is to say, using a PrCo.sub.5 permanent magnet to replace a SmCo.sub.5 permanent magnet is very impractical.
In addition, some people in the art have tried to find ways to lessen the expense of samarium in SmCo.sub.5 permanent magnets by using other light rare-earth elements, such as paraseodymium, as a substitute for part of the samarium as disclosed in Paper No. IV-4 at the 6th International Workshop on Rare-Earth Magnets and their Applications, Baden/Vienna, Austria, Aug. 31-Sept. 3, 1982 by M. Velicescu. However, owing to the unstable intrinsic coercivity of (SmPr)Co.sub.5 magnets, the permanent magnet producers are hardly to get as low a quality loss of the respective magnets as possible in connection with reproducibility, that are comparable with that of SmCo.sub.5 magnets.
Accordingly, it is the primary object of the present invention to provide a process by which a (Sm.sub.x Pr.sub.1-x) Coy (wherein y is 4.6-5.0) magnet having a stable intrinsic coercivity can be obtained.
It is another object of the present invention to provide a process for producing (Sm.sub.x Pr.sub.1-x)Coy (wherein y is 4.6-5.0) magnets having a relatively high intrinsic coercivity.